Moving Subnet from one Active Directory Site to another Site.

We want to test out a new AD Site. We have a site that has a couple of subnets. One of the subnets has about only about 4 users. We want to move this subnet to the new Site. How long does take for the users to start using the domain controller in the new site? How long will it take for the client to switch back incase there are problems in the new sites?